Hello, we have just transitioned out 3 year old to a adult bed - king single - can this be firm against a wall given she is 3?

Do you feel comfortable that there is no risk of entrapment to your baby (between bed & wall)
Is it a bed & mattress or mattress on the floor?
If you are concerned that there could be a risk of entrapment, then keep the mattress away from walls for the time being, then reassess.
https://rednose.org.au/downloads/Cot_to_bed_brochure_for_families.pdf

How has she been going since transferring from cot?